The engineering and scientific study of large-scale systems is now a major focus in technology, biology, sociology, and cognitive science. Deciding where, when, what and how to sense' or measure' is a crucial question in the scientific study of such systems. For example, to understand the nature of the Internet or an ecosystem one would ideally like to monitor it everywhere and all the time. This is impossible, and consequently, engineers and scientists must select a relatively small number of times, places and features for measurement.